1. Field of the Invention
This invention relates to a center pivot irrigation system having the ability to irrigate the corners of the field being irrigated. More particularly, this invention relates to an improved means for controlling a center pivot last regular drive unit and corner span.
2. Description of the Related Art
Conventional center pivot irrigation systems comprise an elongated main boom pivotally connected at its inner end to a center pivot structure and extending outwardly therefrom. The main boom of the conventional center pivot irrigation system is comprised of a plurality of pipes connected together in an end-to-end fashion which are supported upon a plurality of drive towers or units. In most conventional center pivot irrigation systems, the outermost or last regular drive unit is the master unit with the other drive units being selectively driven in response to angular displacement of the boom section adjacent thereto. The last regular drive unit normally includes an electric motor which is either "on" or "off" and is not of the variable speed type.
In the early 1970s, corner systems for center pivot irrigation systems were developed to enable the corners of a square field to be irrigated. See, for example, U.S. Pat. Nos. 3,797,517; 3,802,726; and 3,902,668. Corner systems usually consist of an extension boom or arm, sometimes referred to as a corner span, which is pivotally connected to the outer end of the main boom and which is supported on at least one steerable drive tower or unit. A guidance system is provided for guiding or steering the extension tower, and extension boom, out into the corners of the field as the main arm travels around the field. Perhaps the most popular method of guiding the extension tower is the buried wire system described in U.S. Pat. No. 3,902,668. However, many different types of systems for driving the wheels of the extension tower have been previously provided. See, for example, U.S. Pat. Nos. 4,508,269 and 4,674,681. Further, in assignee's current production model, relative movement between the main boom and the extension boom is sensed with the drive wheels on the steerable drive tower on the extension boom being driven when the main boom has moved forwardly a predetermined distance with respect to the extension boom. When the extension boom "catches up" with the main arm, the driving of the drive wheels on the extension tower is discontinued. This process is continuously repeated during the movement of the system through the field. The connection between the main boom and extension boom is subject to wear due to the relative movement between the spans.
Although the prior art systems do perform generally satisfactorily to drive the corner spans, it is believed that the instant invention is more reliable, more sensitive, and will require less maintenance than the prior art machines.